The main goal of the proposed research is to assess the role of genetic and environmental influences on inhibitory control (IC) in early childhood. In the proposed investigation, 200 twin pairs will be assessed for IC and related behaviors at 2 years of age in a laboratory situation. Parent ratings of IC, behavior problems, and other temperament dimensions, observer ratings of IC and other temperament dimensions, and a measure of cognitive ability, will be obtained. Little is known about the development of IC and related behaviors. Most IC researchers posit that IC emerges at the age of two, however, relatively few have studied it at this important age for methodological reasons. IC's importance in the development of psychopathology, as well as in conjunction with other dimensions of temperament is also an important consideration. Further research can only serve to refine the etiology of IC, and how it contributes to the development of temperament, cognition, and psychopathology in childhood and across the lifespan. The exploration of genetic and environmental contributions, in addition to the possibility of discovering specific genes related to these behaviors, may help predict individuals or groups at risk for developmental difficulties.